Polishing device and method

ABSTRACT

A polishing device includes a polishing head for pressing and holding a semiconductor wafer, a pair of polishing pads having the same diameter as the polishing head for polishing the semiconductor wafer, and a subordinate polishing pad having a smaller diameter for polishing a peripheral portion of the semiconductor wafer. Both the pair of polishing pads are rotated in one direction, or in opposite directions. The rotational speed of the first and/or second polishing pad is controlled so that an equal polishing rate is obtained for the first and second polishing pads when the polishing pads are rotated in opposite directions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to polishing device and method. In particular, the present invention relates to an improvement of the polishing device and method generally used for polishing a wafer.

2. Description of the Related Art

Along with development of a finer design rule in the semiconductor fabrication process, a higher degree of flatness is requested for the surface of a semiconductor wafer in each step of the process for manufacturing semiconductor devices.

For achieving a flat surface of the film on the wafer, attempts have been made to satisfy the requested degree of flatness by improving the flatness of the surface of the as-deposited film itself, and by using a reflow technique such as annealing coated films including a SOG (spin on glass) film and a BPSG (borophospho-silicate glass) film at a high temperature. The CMP technique, which directly polishes the surface of the semiconductor wafer, has been increasingly used since the design rule for the semiconductor device was settled at as low as 0.35 micrometers or smaller.

The CMP process is generally effective for eliminating the step difference on the surface of the semiconductor wafer; however, causes a relatively poor within wafer uniformity in a larger wafer area after the polishing. In recent years, semiconductor devices are fabricated on a larger-diameter wafer having a diameter of 300 mm, for example. This necessitates a further improvement of the within wafer uniformity of the polished film. The polishing device using the CMP process is described, for example, in Patent Publications JP2003-521117A and JP-2001-25962A.

The conventional polishing device will be described with reference to the accompanying drawings. FIGS. 5 and 6 show a sectional view and a top plan view, respectively, of a typical conventional polishing device. The polishing device, generally designated at numeral 100, basically includes a polishing head 11, a polishing pad 15, a dresser 18, and a slurry supply nozzle 17. The polishing head 11 holds a semiconductor wafer 12 while pressing the semiconductor wafer 12 on the rear surface thereof. The polishing pad 15 polishes the main surface of the semiconductor wafer 12. The dresser 18 carries out a dressing treatment of the front surface of the polishing pad 15. The slurry supply nozzle 17 supplies slurry including an abrasive between the polishing pad 15 and the semiconductor wafer 12.

The polishing head 11 presses the semiconductor wafer 12 at the rear surface thereof by using a membrane sheet 14 while retaining the semiconductor wafer 12 in the in-plane direction of the wafer by using a retainer ring 13, whereby rotation of the polishing head 11 rotates the semiconductor wafer 12. The polishing pad 15 polishes the main surface of the semiconductor wafer 12 while, for example, rotating in the same direction as the semiconductor wafer 12. During this polishing treatment, slurry including an abrasive is supplied between the polishing pad 15 and the main surface of the semiconductor wafer 12 from the slurry supply nozzle 17. The slurry spreads uniformly on the front surface of the polishing pad 15 by rotation of the polishing pad 15, whereby the polishing of wafer is performed under the continuous supply of the slurry. If a groove formed on the polishing pad 15 is clogged during the polishing of the wafer, the dresser 18 is activated to perform the dressing treatment on the surface of the polishing pad 15. A periphery pressing member 16 presses the peripheral portion of the wafer 12 to achieve a desired within wafer uniformity, which is likely to be lost particularly in the vicinity of the periphery of the wafer.

The polishing pad 15 is driven for the rotational movement thereof during polishing the wafer, as described above. During the polishing treatment, the center of the polishing pad 15 remains stationary to have a polishing rate of zero. For this reason, the polishing of the semiconductor wafer 12 is performed radially outside the center of the polishing pad 15. Therefore, the diameter of the polishing pad 15 is twice as large as or larger than the diameter of the polishing head 11 which has almost the same diameter as the semiconductor wafer 12. That is, the effective processing area of the polishing pad 15 used for polishing the semiconductor wafer 12 is equal to or less than 25% of the total area of the polishing pad 15.

The smaller ratio of the effective polishing area to the total area of the polishing pad inevitably increases the actual size of the polishing pad, thereby increasing the cost for the CMP process.

In addition, the dresser is used in order to achieve a stable within wafer uniformity of the polishing rate in the CMP process. The larger size of the polishing pad involves a difficulty in achieving the stable dressing treatment and increases the number of times of the dressing treatment. The larger number of times of the dressing treatment reduces the lifetime of the dresser to further increase the cost for the CMP process.

Further, in the CMP process, the polishing pad and the slurry are generally selected depending on the target thin film to be polished. Each time such selection is made, the wafer is transferred to another polishing device which is suited or dedicated to the material of the thin film to be polished. Due to the time length required for such transfer of the wafer, the throughput of the fabrication process of the semiconductor device is lowered. Further, the within wafer uniformity of the film thickness after polishing by the CMP is particularly lower in the vicinity of the periphery of the wafer. Therefore, only an adjustment of the load by using the periphery pressing member in the polishing head is insufficient for achieving the required within wafer uniformity.

SUMMARY OF THE INVENTION

In view of the problem of the conventional polishing device and polishing process as described above, it is an object of the present invention to provide a polishing device and a polishing method which are capable of reducing the cost for the CMP process.

The present invention provides a polishing device including: a polishing head that holds a wafer while allowing rotation thereof; and first and second polishing pads juxtaposed with each other in contact with a surface of the wafer held by the polishing head.

The present invention also provides method for polishing a wafer by using the above polishing device, the method including a first step including concurrent steps of rotating the first polishing pad, supplying slurry onto a surface of the first polishing pad, stopping rotation of the second polishing pad and supplying water onto a surface of the second polishing head.

The present invention further provides a method for polishing a wafer by using the polishing device according to claim 1, the method including a first step including concurrent steps of rotating the first and second polishing pads, and supplying slurry onto a surface of the first and second polishing pads.

The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a polishing device according to an exemplary embodiment of the present invention;

FIG. 2 is a top plan view of the polishing device of FIG. 1;

FIG. 3 is a sectional view showing the state of polishing the wafer at the periphery thereof by using the polishing pad having a small diameter;

FIG. 4 is a top plan view of a polishing device according to a first modification of the above embodiment;

FIG. 5 is a sectional view of a conventional polishing device; and

FIG. 6 is a top plan view of the conventional polishing device.

FIG. 7 is a sectional view of an alignment mark before a polishing treatment.

FIG. 8 is a sectional view of the alignment mark after the polishing treatment using the polishing device of the embodiment.

FIG. 9 is a sectional view of the alignment mark after the polishing treatment using the polishing device according to modifications of the above embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENT

Now, exemplary embodiment of the present invention will be described with reference to accompanying drawings, wherein similar constituent elements are designated by similar reference numerals throughout the drawings.

FIG. 1 is a sectional view of a polishing device (CMP device) according to an embodiment of the present invention, particularly showing the polishing head and the vicinity thereof in the polishing device. FIG. 2 is a top plan view of the CMP device of FIG. 1.

As shown in FIGS. 1 and 2, the polishing device, generally designated at numeral 10, according to the present invention includes the polishing head 11, a pair of main polishing pads 15 and 19, a subordinate polishing pad 22, dressers 18 and 21, slurry supply nozzles 17 and 20, and a polishing pad control unit 23. The polishing head 11 holds thereon a target semiconductor wafer 12 to be polished. The pair of main polishing pads 15 and 19 are made of polyurethane and have a diameter substantially same as the diameter of the polishing head 11. The polishing pad control unit 23 controls the rotational speed, rotational direction and pressing force of the polishing pads 15, 19 and 22. The material of the polishing pad 15 may be different from the material of the polishing pad 19 if it is desired to polish different films made of different materials.

The subordinate polishing pad 22 has a smaller diameter than the polishing head 11 and is used for polishing the peripheral portion of the wafer. The dresser 18 and 21 and the slurry supply nozzles 17 and 20 are attached to the respective polishing pads 15 and 19. The subordinate polishing pad 22 having the smaller diameter and used for polishing the peripheral portion of the wafer 12 have corresponding slurry supply nozzle and dresser, which are omitted for illustration for the purpose of simplification of the drawings. The dresser and the slurry supply nozzle attached to the subordinate polishing pad 22 have a configuration and a function similar to those of the dressers and slurry supply nozzles attached to the pair of main polishing pads 15 and 19. In the polishing device 10 according to the present embodiment, the polishing head 11 rotates, for example, in a clockwise direction, whereas the polishing pads 15, 19, and 22 rotate in a counterclockwise direction, as depicted in these drawings.

The polishing head 11 includes, in addition to a head body including a rotational mechanism, a retainer ring 13 made of polyphenylene sulfide (generally abbreviated as PPS) or polyether-ether-ketone (generally abbreviated as PEEK), a membrane sheet 14 made of neoprene rubber, and a periphery pressing member 16 made of a high-polymer material.

Both the pair of main polishing pads 15 and 19 are configured to have a disk shape, and the top surface thereof is supplied with slurry, which includes an abrasive or is pure water, and is discharged from the slurry supply nozzles 17 and 20. The dressers 18 and 21 have abrasive diamond grains fixed onto the front surface thereof and grind the front surface of the main polishing pads 15 and 19, respectively, each time required to remove irregularity on the front surface of the main polishing pads 15 and 19. From the slurry supply nozzles 17 and 20, the slurry is discharged at a flow rate of, for example, 300 ml./min. (milliliter per minute) The polishing pads 15 and 19 rotate, for example, at a rotational speed of 30 min⁻¹, to supply the discharged slurry to the entire surface of the main polishing pads 15 and 19.

The semiconductor wafer 12 is set on the polishing head 11 in a face-down state. The polishing head 11 rotates at a rotational speed of 29 min⁻¹ integrally with the semiconductor wafer 12. The polishing head 11 moves in a horizontal direction within the radial range of the main polishing pads 15 and 19. The semiconductor wafer 12 is pressed against the main polishing pads 15 and 19 by the polishing head 11 with a mechanical pressing force of F1=70N (Newton). At the same time, the semiconductor wafer 12 is applied with a pressing force of 50N against the main polishing pads 15 and 19, and applied with a pressure (F2) of high-pressure air supplied to a chamber that is defined by the membrane sheet 14 and the bottom surface of the head body.

As shown in FIG. 2, both the polishing pads 15 and 19 have substantially the same size as the polishing head 11, and juxtaposed with each other, substantially without a gap interposed therebetween. Due to such a configuration, the polishing pads 15 and 19 support and retain the semiconductor wafer 12 while each receiving half of the pressing force applied from the semiconductor wafer 12. The moving distance of the polishing head 11 is restricted such that the periphery of the semiconductor wafer 12 does not reach the center of the polishing pad 15 or 19. Polishing treatment of the semiconductor wafer 12 is carried out by both the polishing pads 15 and 19. In the present embodiment, the configuration and function of the polishing pad 15 are similar to those of the polishing pad 19.

The semiconductor wafer 12 is polished by the polishing pad 15 or 19 for a predetermined time length selected in advance. Thereafter, the semiconductor wafer 12 is washed with pure water. The next semiconductor wafer 12 is polished similarly. If the slurry is discharged onto the polishing pad 19 which is not rotated, while the semiconductor wafer 12 is being polished by the polishing pad 15, polishing is carried out also by the polishing pad 19. In order to stop polishing by the polishing pad 19, supply of the slurry onto the polishing pad 19 is stopped. In this case, pure water is discharged onto the polishing pad 19 from the slurry supply nozzle 20. This prevents drying of the polishing pad 19 which is stopped for rotation, and suppresses occurring of a scratch on the surface of the semiconductor wafer 12.

In other words, when the polishing pad 15 rotates and the slurry is supplied onto the polishing pad 15, the polishing pad 19 is stopped and pure water is supplied onto the polishing pad 19. In this manner, the semiconductor wafer 12 is polished by the polishing pad 15 while being supported or held by both the polishing pads 15 and 19. In addition, if polishing is carried out by using the polishing pad 19, similar rotation control and slurry supply control are carried out.

If different slurries are to be used for the main polishing pads 15 and 19, pure water is supplied onto the front surface of one of the polishing pads which stops rotation. If a higher polishing rate is attempted by using both the polishing pads 15 and 19 to polish the wafer 12 at the same time, the slurry is supplied onto both the main polishing pads 15 and 19. The polishing rate is generally in proportion to the F2 pressure with which the semiconductor wafer 12 is pressed against the polishing pad 15 or 19. However, there is a tendency that the within wafer uniformity of the polishing rate is deteriorated especially at the peripheral portion of the wafer. For obtaining a uniform polishing rate, high-pressure air having F3 pressure is supplied to the periphery pressing member 16 shown in FIG. 1, and is adjusted in a range of around 50 +5 N for the pressing force by the periphery pressing member 16.

In the polishing device 10 according to the present embodiment, the subordinate polishing pad 22 having a smaller diameter is mounted between the polishing pad 15 and the polishing pad 19, for performing the polishing treatment of the peripheral portion of the wafer 12. FIG. 3 is a sectional view showing a state of polishing by the subordinate polishing pad 22. The diameter of the subordinate polishing pad 22 is X1=80 mm, for example. At least X2=20 mm is ensured as for the length of the contact area between the subordinate polishing pad 22 and the wafer 12. The subordinate polishing pad 22 ensures this polishing area, and also adjusts the polishing rate for the peripheral portion of the wafer by optimizing the rotational speed.

Although not shown in FIG. 3, for the subordinate polishing pad 22 of the small diameter, similarly to the other polishing pads 15 and 19, slurry is supplied during the dressing treatment by the dresser or the polishing by the polishing pad 22, and pure water is supplied if the polishing treatment is not carried out by the polishing pad 22. In addition, the subordinate polishing pad 22 may be activated for the polishing in synchrony with the polishing pads 15 and 19. For this purpose, the polishing pad control unit 23 shown in FIG. 1 detects the rotational speed of the polishing pads 15 and 19, and based on the detected rotational speed, the rotational speed of the subordinate polishing pad 22 is controlled.

In the polishing device 10 of the present embodiment, the polishing treatment is conducted in the state where the semiconductor wafer 12 is held by both the pair of main polishing pads 15 and 19 due to the above configuration. In the conventional polishing device, the polishing pad having a diameter two times as long as the diameter of the polishing head is used. On the other hand, in the present embodiment, it is sufficient that the pair of main polishing pads 15 and 19 have the diameter same as or similar to the diameter of the polishing head 11. Therefore, the unit cost of the polishing pads is reduced and the total cost necessary for the polishing treatment may be reduced. Further, by arranging the subordinate polishing pad 22 dedicated for polishing the peripheral portion of the wafer, the within wafer uniformity is improved from ±10% that is achieved in the conventional polishing device down to ±5%.

In the above embodiment, an example of the polishing device is described wherein both the polishing pads are used to polish the semiconductor wafer. However, a polishing pad dedicated for supporting the wafer may be adopted as one of the polishing pads. This example is shown in FIG. 4 as a first modification of the above embodiment. A polishing pad 15 has a diameter similar to the diameter of the polishing head 11. Another polishing pad 24 has a smaller diameter and is dedicated for use in supporting the wafer 12. The another polishing pad 24 may have a diameter slightly larger than the radius of the polishing head 11, and the slurry supply nozzle 20 for supplying pure water is arranged on the front surface of the another polishing pad 24 dedicated for use in supporting the wafer 12. The dresser is not provided to the another polishing pad 24. The another polishing pad 24 may be additionally used as the subordinate polishing pad for polishing the peripheral portion of the wafer 12.

In the example of the above embodiment, both the polishing pads 15 and 19 are rotated in the same rotational direction. However, the polishing pads 15 and 19 may be rotated in the opposite directions.

For example, in a second modification of the above embodiment, the polishing pad 15 is rotated in a clockwise direction whereas the polishing pad 19 is rotated in a counterclockwise direction, differently from the configuration shown in FIG. 2. The opposite rotational directions of the polishing pads 15 and 19 may improve the structure of an alignment mark such as used in a photolithographic process, as will be described hereinafter with reference to FIGS. 7 and 8.

FIG. 7 shows the alignment mark in section before the polishing treatment, whereas FIG. 8 shows the alignment mark after the polishing treatment. In FIG. 7, a portion of the wafer in which the alignment mark 34 is to be formed includes an oxide film 31 having therein a depression 32, and a tungsten film 33 covering the oxide film 31 including the internal of the depression 32. The polishing treatment removes a portion of the tungsten film 33 on top of the oxide film 33 to leave the structure of the alignment mark 34 as shown in FIG. 8.

If the polishing pads 15 and 19 are rotated in the same rotational direction, the resultant alignment mark 34 may have a slope 35 on one of the opposing edges of the alignment mark 34, as shown in FIG. 8, the slope 35 falling toward the inner surface of the tungsten film 31. This slope 35 is formed by over-polishing of the polishing pad, and is generally involved with the polishing pad rotating in one direction. The slope 35 formed on one of the opposing edges of the alignment mark 34 may degrade the alignment accuracy in the photolithographic process, due to the loss of symmetry of the alignment mark 34.

In the second modification, the polishing pads 15 and 19 rotating in the opposite directions provide a slope 35 on both the opposing edges of the alignment mark 34, as shown in FIG. 9. This maintains the symmetry for the alignment mark 35, thereby improving the alignment accuracy during the photolithographic process.

If the rotational speeds of both the polishing pads are equal in the second modification, the moving speed of the polishing pad 19 with respect to the wafer surface is smaller than the moving speed of the polishing pad 15 with respect to the wafer surface. This causes different polishing rates in the polishing pads 15 and 19. Thus, the polishing pad control unit 23 controls the rotational speed of the polishing pad 19 so that the polishing rate by the polishing pad 19 is equal to the polishing rate by the polishing pad 15. The control by the polishing pad control unit 23 assures an excellent symmetry in the alignment mark 34, whereby the alignment accuracy is assured in the photolithographic process.

In an alternative, the polishing pad control unit 23 may control, in addition to the rotational speed of the polishing pad 19, the rotational direction, rotational speed and pressure of the dresser 21 dressing the polishing pad 19 based on the rotational speed and pressure of the polishing head 11, which are detected by the polishing pad control unit 23. The control of the rotational direction of the dresser 21 based on the dressing direction of the polishing pad 19 by the dresser 21, together with the control of the rotational speed and the pressure of the dresser 21 removes the clogging of the groove on the polishing pad 19, thereby effectively enhancing the polishing rate by the polishing pad 19.

The polishing pad control unit 23 may control the operation of the polishing pad 15 and the dresser 18 instead of the operation of the polishing pad 19 and the dresser 21, or may control the operation of both the polishing pads 15 and 19 and the dressers 18 and 21.

A third modification of the above embodiment is also an example such that the polishing pads 15 and 19 rotate in opposite directions. With reference to FIG. 2, the first step of the polishing treatment in the third modification includes concurrent steps of rotating the polishing pad 15 in a clockwise direction, supplying slurry onto the polishing pad 15, stopping rotation of the polishing pad 19, and supplying pure water onto the polishing pad 19. The second step includes concurrent steps of stopping rotation of the polishing pad 15, supplying pure water onto the polishing pad 15, rotating the polishing pad 19 in a counterclockwise direction, and supplying slurry onto the polishing pad 19. The term “concurrent steps” as used herein means that those steps overlap each other in at least some interval.

The polishing pad control unit 23 controls the rotational speed of the polishing pad 19, similarly to the second modification, and also controls the rotational direction, rotational speed and pressure of the dresser 21. In the third modification, the opposite rotational directions of the polishing pads 15 and 19, the control of the rotational speed of the polishing pad 19 and the control of the rotational direction, rotational speed and pressure of the dresser 21 provide an excellent symmetry for the alignment mark 35 as shown in FIG. 9, similarly to the case of the second modification.

In the example of the above embodiment and first through third modifications, the polishing head 11 is rotated in one direction during the entire polishing treatment. However, the rotational direction of the polishing head 11 may be reversed during the polishing treatment.

In a fourth modification, for example, a first step uses the step described for the second modification, and a subsequent second step uses a counterclockwise rotation of the polishing head 11 together with a step similar to the first step.

In a fifth modification, first through fourth steps uses the steps described in the third modification with or without a modification. More specifically, the first step includes concurrent steps of rotating the polishing head in a clockwise direction, rotating the polishing pad 15 in a clockwise direction, supplying slurry onto the polishing pad 15, stopping rotation of the polishing pad 19, and supplying pure water onto the polishing pad 19. The second step includes concurrent steps of rotating the polishing head in a clockwise direction, stopping rotation of the polishing pad 15, supplying pure water onto the polishing pad 15, rotating the polishing pad 19 in a counterclockwise direction, and supplying slurry onto the polishing pad 19. The third step includes concurrent steps of rotating the polishing head in a counterclockwise direction, stopping rotation of the polishing pad 15, supplying pure water onto the polishing pad 15, rotating the polishing pad 19 in a counterclockwise direction, and supplying slurry onto the polishing pad 19. The fourth step includes concurrent steps of rotating the polishing head in a counterclockwise direction, rotating the polishing pad 15 in a clockwise direction, supplying slurry onto the polishing pad 15, stopping rotation of the polishing pad 19, and supplying pure water onto the polishing pad 19.

The slope 35 of the alignment mark 34 is formed by the movement of the polishing surface of the polishing pads 15 and 19 with respect to the wafer surface. More specifically, the shape of the slope 35 depends on the rotational direction of the polishing head 11, in addition to the rotational direction of the polishing pads 15 and 19. In view of this fact, in a fifth modification, the effect on the wafer surface caused by the movement of the polishing surface of the polishing pads 15 and 19 in one direction is cancelled by the effect on the wafer surface caused by the movement of the polishing surface in the opposite direction. This provides a uniform polishing rate on the opposite edges of the alignment mark 34, to further improve the symmetry of the alignment mark 34.

It should be noted that the order of the steps described in each of the third through fifth steps may be changed as desired.

In the above embodiment and the modifications, since the size of the polishing pads in the embodiment is reduced compared to the conventional polishing pad, the unit cost for the polishing pads is reduced, whereby the total cost for the polishing treatment may be reduced, substantially without a reduction in the polishing rate.

Use of one of the polishing pads for the polishing treatment while stopping rotation of the other of the polishing pads, if employed, allows the one of the polishing pads having a smaller size to polish the wafer while supporting the wafer by the other of the polishing pads.

While the invention has been particularly shown and described with reference to exemplary embodiment and modifications thereof, the invention is not limited to these embodiment and modifications. It will be understood by those of ordinary skill in the art that various changes in form and details be made therein without departing from the spirit and scope of the present invention as defined in the claims. 

1. A polishing device comprising: a polishing head that holds a wafer while allowing rotation thereof; and first and second polishing pads juxtaposed with each other in contact with a surface of the wafer held by said polishing head.
 2. The polishing device according to claim 1, said polishing head has a diameter substantially equal to a diameter of at least one of said first and second polishing pads.
 3. The polishing device according to claim 2, wherein said at least one of said first and second polishing pads include both said first and second polishing pads.
 4. The polishing device according to claim 2, wherein said first polishing pad has the diameter substantially equal to the diameter of said polishing head, and said second polishing pad has a diameter smaller than the diameter of said polishing head.
 5. The polishing device according to claim 1, further comprising a third polishing pad juxtaposed with said first and second polishing pads in contact with the surface of the wafer held by said polishing head, wherein said third polishing pad polishes a peripheral area of the surface of the wafer.
 6. The polishing device according to claim 1, wherein said first polishing pad includes a material different from a material of said second polishing pad.
 7. The polishing device according to claim 1, further comprising a polishing pad control unit that controls a rotational speed of said first polishing pad and/or said second polishing pad so that said first and second polishing pads have an equal polishing rate if said first and second polishing pads rotate in opposite directions.
 8. The polishing device according to claim 7, further comprising first and second dressers that dress said first and second polishing pads, respectively, wherein said polishing pad control unit further controls a rotational direction, rotational speed and pressure of at least one of said first and second dressers.
 9. A method for polishing a wafer by using the polishing device according to claim 1, said method comprising a first step including concurrent steps of rotating said first polishing pad, supplying slurry onto a surface of said first polishing pad, stopping rotation of said second polishing pad and supplying water onto a surface of said second polishing head.
 10. The method according to claim 9, further comprising a second step including concurrent steps of stopping rotation of said first polishing pad, supplying water onto a surface of said first polishing pad, rotating said second polishing pad, and supplying slurry onto a surface of said second polishing pad.
 11. The method according to claim 10, wherein a rotational direction of said first polishing pad in said first step is opposite to a rotational direction of said second polishing pad in said second step.
 12. The method according to claim 11, further comprising a third step including said consecutive steps of said first step, and a fourth step including said consecutive steps of said second step, wherein a rotational direction of said polishing head is different between said first step and said third step and between said second step and said fourth step.
 13. A method for polishing a wafer by using the polishing device according to claim 1, said method comprising a first step including concurrent steps of rotating said first and second polishing pads, and supplying slurry onto a surface of said first and second polishing pads.
 14. The method according to claim 13, wherein said first and second polishing pads rotate in opposite directions.
 15. The method according to claim 14, wherein a rotational direction of said polishing head is reversed during said first step. 